Optical Kerr Effect Experiments on Complex Liquids

OPTICAL KERR EFFECT EXPERIMENTS ON COMPLEX LIQUIDS 

Paolo Bartolini, Andrea Taschin, Roberto Eramo, and Renato Torre 

Abstract The time-resolved spectroscopy based on polarization effects represents one of the most sensitive techniques for studying dynamical phenomena in condensed matter. The optical Kerr effect performed with ultra-short laser pulses enables a unique investigation of dynamic processes covering a wide time range, typically from few femtoseconds up to many nanoseconds. This spectroscopic tool is particularly well suited for the measurement of relaxation patterns in complex liquids where several dynamic phenomena, taking place on different time scales, are present. In this chapter we introduce the optical Kerr effect principles, the experimental procedure, and some results from measurements in a number of different complex liquids. 

Since 1875, thanks to Kerr s discovery [ 1], it is known that a static electric field can induce a modification of the optical properties of a liquid. Many years later researchers found out that also an optical electromagnetic field was capable of producing a measurable modification of the dielectric properties, inducing a hirefiringence effect the first experimental observation of the optical Ken-effect (OKE) was reported in 1963 [2]. After few years, with the introduction of the first pulsed lasers, spectroscopists discovered the chance to induce in a material a transient birefringence and to measure its relaxation toward the equiUhrium [3]. They also realized that this could be a relevant new spectroscopic tool able to collect new information on the dynamical processes present in the material. The spectroscopic research, worked out in the following years, confirmed this forecast beyond the expectations. Two important experimental improvements of this spectroscopic technique have been made. On one hand, the pulsed laser sources have become able to produce very short pulses of high 

When a laser pulse passes through a material it produces a local non equilibrium state that induces a modification of the optical properties. This is a transient effect that relaxes back to the equilihrium state through a variety of processes. In a typical pump-prohe experiment, a second laser pulse is sent on the material probing the optical modifications induced by the pump pulse. Since the second laser pulse arrives with a controlled delay, it monitors and measures the transient optical excitation and hence the relaxation of the nonequilibrium state. In the time-resolved OKE both the pulses, pump and probe, are linearly 

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Optical Kerr Effect Experiments on Complex Liquids... 

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